The ionosphere is the ionized part of the atmosphere produced primarily by the absorption of solar radiation. It plays an important part in upper atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio wave propagation to distant places on the Earth. The influence extends across a wide range of radio frequency bands, well above the high frequency band, considered to be 3-30 MHz.
The F-region, also known as the Appleton layer, is approximately 120 km to 525 km above the surface of the Earth. Here extreme ultraviolet (UV) (10-100 nm) solar radiation ionizes atomic oxygen (O). The F-region is the most important part of the ionosphere in terms of high frequency (HF) communications. The F-region combines into one layer at night, and in the presence of sunlight (during daytime), it divides into two layers, the F1 and F2. The F-region is responsible for most skywave propagation of radio waves, and is thickest and most reflective of radio waves on the side of the Earth facing the sun. The amount of electrons in the F-region is the key parameter affecting radio communications. In the F-region, the amounts of electrons and O+ ions are virtually identical. Accordingly, it is important to monitor the amount of atomic oxygen ions (O+) in the F-region of the ionosphere to compensate for negative effects on such signals.
Satellite-borne remote sensing of the ionospheric F-region proposes disk-viewing dayside observations of 83.4 nm emissions by atomic oxygen ions (O+). FIG. 1 illustrates such a system. In the figure a satellite 108 has a line of sight to the earth 106 along a vector 110, which passes through the top of F-region 102 and the bottom of F-region 104. By conventional methods, satellite 108 is able to detect a total of emissions by atomic oxygen ions (O+) within and below the F-region or the portion of vector 110 that lies below 102. The spectroscopic notation for the dominant atomic oxygen ion (O+) emission related to the ionosphere is “O II 83.4 nm,” where the Roman numeral “II” specifies an emission from singly ionized atomic oxygen, or O+. Emissions from a neutral atomic species would use the Roman numeral I. Here the term “disk-viewing” implies any measurement from above the Earth's surface in which the instrument line of sight intersects the surface of the Earth, for example, vector 110, even when the instrument field of view is not large enough to permit simultaneous imaging of the entire disk.
What would be more valuable for radio wave communications is an altitude profile [O+](z) of the amount of atomic oxygen ions (O+) at each altitude z. In other words, in addition to the total amount of emission along vector portion 112, an altitude function [O+](z) of the O+number density along vector portion 112 would be valuable. A mapping of such altitude functions along an area of the earth would greatly enable HF communication systems to compensate for negative effects of atomic oxygen ions on HF and higher frequency signals.
Unfortunately, in the scientific community, there is a conventionally perceived insurmountable obstacle to disk imaging of the dayside ionosphere F-region using the 83.4 nm airglow along vector portion 112. It is conventionally accepted that such images are impossible to interpret because the information retrieval problem is severely underdetermined (more variables than equations) and because the photons undergo resonant scattering in the F-region causing increased optical path length (reduced signal). To understand this problem more clearly, the production of 83.4 nm photons and the propagation of those photons through the F-region should be discussed in relation to the intensity of 83.4 nm emissions measured by the satellite instrument in a specific disc-viewed pixel.
As discussed above, ionospheric parameters define an altitude profile of [O+](z), where z is the altitude above earth at the geodetic latitude and longitude of the observation by a downward-pointing (or disk-viewing) space-borne spectrograph, spectrometer, or photometer, i.e., the F-region along vector portion 112. These parameters define an analytic profile, known as a Chapman layer:
                                          [                          O              +                        ]                    ⁢                      (            z            )                          =                              N            max                    ⁢                                    exp              ⁡                              [                                                      1                    2                                    ⁢                                      (                                          1                      -                                                                        z                          -                                                      z                            max                                                                          H                                            -                                              exp                        ⁢                                                  {                                                      -                                                                                          z                                -                                                                  z                                  max                                                                                            H                                                                                }                                                                                      )                                                  ]                                      .                                              (        1        )            
FIG. 2 illustrates an exemplary Chapman layer or the F-region profile [O+](z) of the density of O+ ions and the 83.4 nm source region as function of atmospheric altitude for a specific disc-viewed pixel. Notice that in FIG. 2, the vertical axis corresponds to the independent variable, z, and the horizontal axis represents the function [O+](z). As illustrated in the figure, the F-region 202 is bounded on top and bottom by dashed lines corresponding to 102 and 104 on FIG. 1. Area 204, above F-region 202, comprises more hydrogen ions (H+) ions than oxygen ions (O+). As such, the amount of O+ ions is greatest at points in F-region 202, as represented by curve 208. Point 214 on curve 208 is the altitude, zmax, corresponding to the peak oxygen ion density Nmax. Density curve 208 is a function of altitude, and is based on the altitude at which the O+ number density peaks, the peak O+ number density and the neutral atomic oxygen scale height, H, which determines the shape of the curve. As seen on the right-hand side of Equation (1) above, the second term in the parenthesis determines the decrease in O+ number density as altitude increases, where z>zmax, and corresponds with the gradient of portion of curve 208 that is labeled 212. The third term in parenthesis is causing a rapid decrease in O+ number density as altitude decreases, where z<zmax corresponds with the gradient of portion of curve 208 that is labeled 216. The primary source of 83.4 nm emissions, curve 210, occurs primarily at altitudes below zmax. There the sun ionizes neutral atomic oxygen. Source region 210 extends below the F-region as shown. The satellite detector counts 83.4 nm photons that propagate upward from source region 210. This signal is reduced as the photons are scattered out of the instrument line of sight by the oxygen ions (O+) in the F-region. This reduction is greater (the 83.4 nm signal is weaker) when the amount of O+ is greater, that is, when Nmax (at point 214) is greater, or when H (label 212) is greater, causing the oxygen ion density curve to drop off more gradually with altitude z above zmax. The reduction is less (the 83.4 nm signal is stronger) when zmax is lower, causing greater overlap of the F-region, 208, with the source region 210. Thus the measured signal depends directly on the altitude profile [O+](z), which is characterized by Nmax, zmax, and H. Note that H can be a function of altitude, z, introducing additional parameters into Equation (1) for the O+ number density profile.
If the altitude, Zmax, of the O+ number density peak, the peak O+ number density. Nmax, and the neutral atomic oxygen scale height, H, are known, then the F-region oxygen ion density profile [O+](z) 208 is known, and the 83.4 nm intensity value along the vector 110 may be derived. With a derived intensity value and the detected 83.4 nm emission intensity, it would be possible to derive the altitude profile [O+](z) of the amount of atomic oxygen ions (O+). Hence, it would be possible to map a plurality of such altitude profiles over an area of the earth to greatly enable HF communication systems to compensate for effects of atomic oxygen ions on HF signals.
Unfortunately, conventional methods and systems are unable to accurately determine zmax, Nmax, and H of the dayside ionosphere in the F-region using the 83.4 nm airglow along vector 110. Specifically, the retrieval problem is severely underdetermined; i.e., each line-of-sight observation (or each pixel of an image) along vector portion 112 produces one number, which contains partial information on the several parameters that are needed to specify realistically the associated altitude profile [O+](z) of the O+ number density. Denote by nm the number of ionospheric parameters required to estimate realistically the ionospheric altitude profile [O+](z) within a specified altitude range and at a specified latitude, longitude, and time. As discussed above, the value of nm depends on the shape and peak value of [O+](z) in the region of interest. For the ionospheric F-region, experimental studies have shown that nm≧3. Without at least nn−1 additional, relevant, independent numbers to supplement each disk-viewing airglow measurement, a unique quantitative estimate of [O+](z) in the observed region is not possible. This obstacle has severely impeded the development of extreme ultraviolet disk-viewing systems for monitoring the dayside ionosphere or measuring dayside ionospheric parameters quantitatively.
What is needed is a method and system to accurately determine the ionospheric altitude profile [O+](z) in the F-region using the 83.4 nm airglow along a vector from a satellite to the earth.